The present invention relates to lubrication of moving parts in a compressor. More specifically, the present invention pertains to friction reducing coatings for compressor parts.
To reduce friction between members that form the internal mechanism of a swash plate compressor, various technologies for coating the sliding surfaces of the members have been proposed.
Japanese Unexamined Patent Publication No. 57-146070 describes a double-head-piston-type compressor, the swash plate angle of which is fixed. In the compressor, the spherical surfaces of the shoes for coupling the periphery of the swash plate to the pistons are coated with a lubricant film containing solid lubricant. The coating reduces frictional resistance between the spherical surfaces of the shoes and the corresponding recessed surfaces of the pistons, which reduces power losses.
Japanese Unexamined Patent Publication No. 8-247026 also describes a double-head-piston-type compressor, the swash plate angle of which is fixed. In the compressor, the recessed surfaces of the pistons for receiving the spherical surfaces of the shoes (also known as cam followers) are coated with a film that is mainly made of tin. The tin coating reduces friction between the spherical surfaces of the shoes and the recessed surfaces of the pistons, which prevents damage to the surfaces caused by heat.
These two prior art references relate to fixed swash plate/fixed displacement type compressors, which have fixed swash plate angles. Another type of compressor is known as a variable displacement type. The swash plate of a variable displacement compressor is connected to the drive shaft and is permitted to incline. The swash plate angle .theta. (the inclination angle of the swash plate with respect to an imaginary plane P perpendicular to the drive shaft) ranges from a minimum inclination angle .theta.min to a maximum inclination angle .theta.max, which varies the piston stroke and the displacement of the compressor.
In particular, in the field of air-conditioners for vehicles, variable displacement swash plate compressors that vary the displacement in accordance with the cooling load achieve advantages that cannot be achieved by other types of compressors.
In a typical swash plate compressor, the piston stroke (displacement) is determined in accordance with the swash plate diameter (diameter of an imaginary circle that passes through the centers of the piston couplings) and the swash plate angle. The maximum inclination angle, at which the displacement of the compressor is maximized, is determined in consideration of the permissible limit of friction between the swash plate and the shoes and between the shoes and the pistons during the rotation of the drive shaft and the swash plate. In other words, the permissible limit of friction between the sliding members related to the swash plate is the factor that determines the maximum inclination angle. However, in a swash plate compressor, lubricant oil retained in the compressor is atomized by gas (refrigerant gas such as a chlorofluorocarbon) that circulates in the compressor and is carried to the moving parts. Lubrication and friction in the internal mechanism of a compressor are not problematic as long as the compressor is operating normally.
However, in addition, a coating, as in the prior art mentioned previously, is necessary since there are times when the lubrication by atomized oil is not reliable. That is, there may be a temporary shortage of lubricant oil. For example, when the compressor is started after not operating for a long time, the supply of oil may be inadequate. This is because refrigerant gas is liquefied after the compressor is stopped, and the liquefied refrigerant gas washes away lubricant oil from the moving parts. Accordingly, the parts are not lubricated well when the compressor is started. It takes about one minute until lubricant oil is supplied to the moving parts again by oil atomized by refrigerant gas that enters the compressor. During the one-minute period after the compressor starts, the moving parts that need lubrication are not supplied with oil. Certain parts are coated to provide minimum lubrication in this period. In a conventional variable displacement swash plate compressor (the maximum inclination angle of which is around 19 degrees), the problem of limited lubrication has been solved by taking the prior art measures described previously.
However, in recent years, smaller compressors having larger displacements have been required because of the increasing demand for saving energy and space. Accordingly, it is not acceptable to increase the maximum displacement of a compressor by increasing the swash plate diameter and the housing size. Therefore, it is necessary to increase the piston stroke by increasing the maximum inclination angle of the swash plate. It is empirically known that the maximum inclination angle is limited to around nineteen degrees and cannot be increased more than that with only the prior art coating measures described previously. Therefore, there is a need for a better way to reduce the friction between the spherical surfaces of the shoes and the recessed surfaces of the pistons during the first minute of operation.